The Mad Scientists at DARPA Have a Crazy Idea: Warplanes That See Through Clouds

Despite big advancements in sensor technology in recent years, clouds can still block a warplane crew’s view — and make it impossible to support troops on the grounds.

Synthetic aperture radars can peer through weather, but these sensors generally are too big and unwieldy to help in a fast-evolving close-air-support scenario.

That could change soon. The U.S. Defense Advanced Research Projects Agency, or DARPA, has tested a compact video synthetic aperture radar, or ViSAR, that can peer through clouds and pump radar-based full-motion video to a cockpit or crew station.

The need is clear. “In many important parts of the world (such as the Korean Peninsula, Central America, Colombia and the Balkans) clouds are present between 25 and 50 percent of the time,” Bruce Wallace, a DARPA researcher, wrote in 2015. “The amount of time in which U.S. close-air-support aircraft can engage targets is therefore severely limited.”

DARPA launched the ViSAR development effort in 2013. “Our radar system can be used to image the ground, even through clouds and dust, at a sufficiently high resolution and frame rate to support the engagement of maneuvering targets,” Wallace wrote. The developers want ViSAR to be small enough to fit in the same kind of compact gimbals that, today, contain electro-optical and infrared sensors.

Work focused on a few key components — compact EHF-band exciters, receivers and amplifiers and associated software algorithms. The agency settled on the 233GHz frequency for ViSAR. Wallace explained that the frequency represents a balance between frame-rate, resolution and antenna-size.

But the frequency also posed problems. “For the design and development of our prototype ViSAR system, there were no suitable existing electronics for use in the 233GHz band,” Wallace wrote. “We have therefore fabricated the necessary hardware ourselves.”

On Sept. 28, 2017, the agency announced that a prototype ViSAR system had completed initial flight-testing. “The recent ViSAR tests took place on a modified DC-3 aircraft that flew at low and medium altitudes, allowing researchers to collect and compare data from the ViSAR, E.O. and I.R. sensors mounted on standard sensor gimbals,” DARPA stated.

Wallace indicated that, for early ViSAR tests, DARPA would use the 20-inch-diameter Multi-Spectral Targeting System-B gimbal that’s also installed on the MQ-9 drone and other warplanes. “By fitting with this gimbal, we plan to demonstrate our ViSAR system can easily be installed on tactical aircraft,” Wallace wrote.

Next, researchers will work on refining the sensor’s visualization software “to provide operators a representation they’re used to seeing,” Wallace said. “We don’t want operators in the back of an aircraft to need special radar training to interpret the sensor’s data—we are working to make the visual interface as easy to interpret as existing E.O./I.R. sensor displays.”

At the same time, DARPA will integrate ViSAR onto a representative combat aircraft with an operational battle-management system — as opposed to the rudimentary, non-combat DC-3 test plane. Wallace has hinted that the AC-130 gunship could be the main beneficiary of an operational ViSAR system.

But if development succeeds and the military chooses to acquire ViSAR, any warplane with a sensor gimbal — including drones, Special Operations Forces transports, F-35 stealth fighters and maritime patrol aircraft — could carry the radar alongside E.O. and I.R. sensors. It should also be possible to fit ViSAR in a sensor pod.

ViSAR should function seamlessly as part of a sensor suite. In clear conditions, the E.O. sensor would feed video to the crew. At night, the operators would switch to the I.R. sensor. And when clouds or dust obscure the ground, ViSAR could take over. To the crew, it would all look like video.

While ViSAR’s main application will be “over-the-air real-time imaging of moving and stationary targets through clouds,” Wallace wrote that he and his colleagues are also exploring other possible uses. “These could include the measurement of wind speeds and directions (to inform adjusting fire), performing battle damage assessments and providing a secure short-range air-to-ground datalink.”